I had low vitamin D levels for 3 years after moving to the pacific northwest in Oregon from San Diego. Life was nothing short of a nightmare with my mood and coming out of being a vegan previously for 13 years.
I started taking 5,000iu D3 with Magnesium Glycinate. After that my levels went from the 30's and 40's up to 87. My understanding is that for men it should be in the 75 to 100 range.
I can't say that I'm where I want to be yet in regards to my mood/energy/strength. And I've definitely noticed improvements in my health after I got up into those levels with the D3 and Magnesium Glycinate.
Seems to me like many things are specific to the individual where this may be true it's worthless for one and helpful for another.